1. Field of the Invention
This invention relates to methods and apparatus for curing the coatings of optical fibers.
2. Background Art
The concept of this invention arises from experience in drawing optical fiber that demonstrates that the ultraviolet-initiated curing of the optical fiber coatings (inner, or primary coating and outer, or secondary coating) becomes difficult at high draw speeds, particularly so for the primary coating. When drawing fiber at increased speed, the reduced time in the UV irradiators (reducing the total UV dose) results in incomplete cure of the fiber coatings, particularly the soft primary coating. The first, most obvious reaction to this problem is to add UV lamps to bring the UV dose back to the level used at slower draw speed. Experience shows that this approach is not efficient in improving the cure of the primary coating at higher draw speed. There is not a simple, linear correlation between number of UV irradiators and primary coating degree of cure.
The modulus (basically the stress per unit strain at small deformations) of the primary coating is proportional to the degree to which the acrylate functional groups have reacted during cure. The reaction ties the pre-polymer molecules together through the reactive acrylate groups to build a crosslinked network. The cross-linking builds the modulus. Therefore, it is convenient to measure the modulus of the primary coating on fiber to determine the relative degree of cure of the primary coating. For primary coating cured on fiber under typical draw conditions, the modulus is typically not more than about half the modulus of the same material cured in thin films in the laboratory. One of the differences between the lab curing apparatus and the UV irradiator configuration necessary for the fiber draw towers is that the environment of the fiber in the irradiators during draw is much hotter than that of the films cured in the lab.
FIG. 1 shows the interior setup of the UV irradiator on a draw tower. The UV bulb 1 is backed by an elliptical reflector 2 positioned so that the bulb is in one of the foci of the ellipse partly defined by the reflector. The fiber F passes in front of another elliptical reflector 3 and through the other focus of the ellipse defined by the reflectors. The fiber goes through a quartz center tube 4 that is necessary to protect the fiber from the strong cross-flow of air (not shown) cooling the UV bulb (which is extremely hot). Nitrogen passes up through the center tube (in the direction indicated by arrows in FIG. 1) to provide an inert atmosphere that is conducive to good surface cure of the secondary coating.
There is a strong infrared component to the output irradiation from the UV bulb 1. This is reflected to the fiber as well as being partly absorbed by the center tube 4 (silica has a strong and broad absorbance band in the infrared). The focused irradiation and the heat from the center tube result in an increased temperature of the coating during cure, with the energy adding to the temperature rise from the exothermic cure reaction itself The high intensity UV in excess also raises the temperature of the coating. There is little opportunity for the curing coating effectively to throw off the heat until the coated fiber has exited the UV irradiators, at which point photoinitiation ceases.
An experiment was conducted to test the effects of removing the heat during cure on a draw tower. The center tube 4 in FIG. 1 was simply removed, and fiber was drawn and coated. The cross-flow of air cooling the UV bulb now also cooled the fiber. The result was that the modulus of the primary coating was made to match that of films of the coating cured in the lab. That is, the cure of the primary coating was near 100 percent.
UV lamps are available now that have reflector technology capable of removing the IR component from the spectrum of irradiation impinging on the fiber coatings. This has been shown to be of significant benefit to cure. An example of this technology is manufactured by the Iwasaki Corporation and marketed in the US by Eye Ultraviolet in Massachusetts. This company also has UV bulbs that are water-jacketed, so that much of the IR component is absorbed by water interposed between the UV generating plasma column of the bulb and the coated fiber.
Overton and Taylor (U.S. Pat. No. 4,913,859) disclose achieving an effect similar to the water jacketed bulb by using a water jacket around the center tube in the irradiator to absorb most of the infrared component of the energy impinging on the fiber coating while letting the UV component pass through. This is effective in somewhat reducing the temperature rise in the coatings due to the irradiation. However, the excess UV energy and the increase in temperature due to the chemical reaction (the exotherm) still affect the cure speed.
Mensah and Powers (U.S. Pat. No. 4,636,405) disclose the same kind of water-jacketed center tube to reduce the temperature of the coating, but expressly for the purpose of controlling coating defects induced when excessively warm primary coating on the fiber enters the cool secondary coating applicator. The mismatch in temperature causes the secondary coating prepolymer to adhere poorly to the primary coating already on the fiber, and the secondary coating applies with lumps and neckdowns and bubbles that must be cut from the fiber later.
Prior work in the field has shown that elevated temperature can retard the development of the desired cross-link structure of the optical fiber primary coating1,2. No other commercial UV curable material has been reported as behaving this way, because the optical fiber primary coating is singular among UV-curable materials in its physical properties (being very soft and with such a low glass transition temperature). The optical fiber secondary coating, for example, typically shows an increased rate of polymerization at higher temperature. Two mechanisms unique to the primary coating account for the effect of high temperature on its polymerization rate.
1B. J. Overton, C. R. Taylor, A. J. Muller, Polymer Engineering and Science 29, 1165, (1989) 
2H. Takase, Y, Hashiguchi, Y. Takasugi, N. Saito, T. Ukachi, Proceedings of the International Wire and Cable Symposium, 72, 1994 
The composition of an optical fiber coating pre-polymer comprises an oligomer that is end-capped with, for example, acrylate functional groups. Other reactive species may be used, such as vinyl ether groups. The functional groups are capable of reacting through a free-radical mechanism. The oligomer is of high viscosity and must be diluted to allow application to fiber in the draw process. In order to avoid solvents for dilution, low viscosity monomers containing reactive functional groups are used. The monomers may contain one or more reactive groups. If a monomer contains only one reactive group, it does not add to the cross-link density of the cured material. If it contains more than one reactive group per monomer molecule, it adds to the cross-link density and thus to the modulus of the cured material.
In order to obtain soft, buffer materials useful for primary coatings (where the room temperature Young's modulus is normally less than about 3 MPa on fiber), the oligomers are usually of much higher molecular weight than those used for tough, hard secondary coatings. The oligomer backbone chemistry is chosen to be highly flexible rather than rigid in nature. Additionally, mono-functional diluents are substituted for multi-functional diluents. These choices of raw materials for primary coatings result in the low cross-link density and flexibility necessary to buffer the fiber against stresses from external sources and to remain soft at low temperatures for protection against temperature-induced stresses that cause microbending in the fiber and loss of signal. At the same time, these deliberate choices in raw materials result in primary coating prepolymer mixtures that are quite low in reactive functional group concentration by comparison with the optical fiber secondary coating prepolymer or with virtually any other UV curable prepolymer in use for any other application. The effect of the low reactive functional group concentration and the high molecular weight or long-chain oligomers in primary coating prepolymers is to isolate the photoinitiator in molecular “cages” where they are inefficient in starting the polymerization reaction to create the cross-linked network of the primary coatings. The photoinitiator, once activated by absorption of the UV energy, is likely to react with itself before initiating the desired reaction, due to its relative isolation. Further, since the propagation rate of the polymerization process is directly proportional to the concentration of reactive functional groups, this step in the cure of primary coatings is slower than it is in the cure of other photoreactive materials. Thus, optical fiber primary coatings are inherently quite difficult to cure adequately in fiber draw processes.
The effect of high temperature in retarding the development of the cross-linked structure also appears to be related to the low concentration of reactive functional groups. Because of the scarcity of reactive groups, the free radicals existing in the propagation stage of coating cure are more likely to terminate through recombination with other free radicals, either photoinitiator fragments or propagating chains, or to undergo chain transfer of the radicals on a non-productive course. Therefore, it is beneficial to keep the temperature of the primary coating from rising significantly while the coating is curing.
Since the cure reaction proceeds at an extremely high rate, it is necessary to take steps to remove the heat from the primary coating during irradiation in order to avoid taking additional space on draw towers to cool and re-irradiate the coating repeatedly to achieve adequate cure. There currently is no solution available that addresses this need in a satisfactory manner or degree.